HFR technologies, such as the Spectral Band Replication (SBR) technology, allow to significantly improve the coding efficiency of traditional perceptual audio codecs. In combination with MPEG-4 Advanced Audio Coding (AAC) HFR forms a very efficient audio codec, which is already in use within the XM Satellite Radio system and Digital Radio Mondiale, and also standardized within 3GPP, DVD Forum and others. The combination of AAC and SBR is called aacPlus. It is part of the MPEG-4 standard where it is referred to as the High Efficiency AAC Profile (HE-AAC). In general, HFR technology can be combined with any perceptual audio codec in a back and forward compatible way, thus offering the possibility to upgrade already established broadcasting systems like the MPEG Layer-2 used in the Eureka DAB system. HFR methods can also be combined with speech codecs to allow wide band speech at ultra low bit rates.
The basic idea behind HFR is the observation that usually a strong correlation between the characteristics of the high frequency range of a signal and the characteristics of the low frequency range of the same signal is present. Thus, a good approximation for the representation of the original input high frequency range of a signal can be achieved by a signal transposition from the low frequency range to the high frequency range.
This concept of transposition was established in WO 98/57436 which is incorporated by reference, as a method to recreate a high frequency band from a lower frequency band of an audio signal. A substantial saving in bit-rate can be obtained by using this concept in audio coding and/or speech coding. In the following, reference will be made to audio coding, but it should be noted that the described methods and systems are equally applicable to speech coding and in unified speech and audio coding (USAC).
High Frequency Reconstruction can be performed in the time-domain or in the frequency domain, using a filterbank or transform of choice. The process usually involves several steps, where the two main operations are to firstly create a high frequency excitation signal, and to subsequently shape the high frequency excitation signal to approximate the spectral envelope of the original high frequency spectrum. The step of creating a high frequency excitation signal may e.g. be based on single sideband modulation (SSB) where a sinusoid with frequency ω is mapped to a sinusoid with frequency ω+Δω where Δω is a fixed frequency shift. In other words, the high frequency signal may be generated from the low frequency signal by a “copy-up” operation of low frequency subbands to high frequency subbands. A further approach to creating a high frequency excitation signal may involve harmonic transposition of low frequency subbands. Harmonic transposition of order T is typically designed to map a sinusoid of frequency ω of the low frequency signal to a sinusoid with frequency Tω, with T>1, of the high frequency signal.
The HFR technology may be used as part of source coding systems, where assorted control information to guide the HFR process is transmitted from an encoder to a decoder along with a representation of the narrow band/low frequency signal. For systems where no additional control signal can be transmitted, the process may be applied on the decoder side with the suitable control data estimated from the available information on the decoder side.
The aforementioned envelope adjustment of the high frequency excitation signal aims at accomplishing a spectral shape that resembles the spectral shape of the original highband. In order to do so, the spectral shape of the high frequency signal has to be modified. Put differently, the adjustment to be applied to the highband is a function of the existing spectral envelope and the desired target spectral envelope.
For systems that operate in the frequency domain, e.g. HFR systems implemented in a pseudo-QMF filterbank, prior art methods are suboptimal in this regard, since the creation of the highband signal, by means of combining several contributions from the source frequency range, introduces an artificial spectral envelope into the highband to be envelope adjusted. In other words, the highband or high frequency signal generated from the low frequency signal during the HFR process typically exhibits an artificial spectral envelope (typically comprising spectral discontinuities). This poses difficulties for the spectral envelope adjuster, since the adjuster not only has to have the ability to apply the desired spectral envelope with proper time and frequency resolution, but the adjustor also has to be able to undo the artificially introduced spectral characteristics by the HFR signal generator. This poses difficult design constraints on the envelope adjuster. As a result, these difficulties tend to lead to a perceived loss of high frequency energy, and audible discontinuities in the spectral shape in the highband signal, particularly for speech type signals. In other words, conventional HFR signal generators tend to introduce discontinuities and level variations into the highband signal for signals which have large variations in level over the lowband range, e.g. sibilants. When subsequently the envelope adjuster is exposed to this highband signal, the envelope adjuster cannot with reasonability and consistence separate the newly introduced discontinuity from any natural spectral characteristic of the low band signal.
The present document outlines a solution to the aforementioned problem, which results in an increased perceived audio quality. In particular, the present document describes a solution to the problem of generating a highband signal from a lowband signal, wherein the spectral envelope of the highband signal is effectively adjusted to resemble the original spectral envelope in the highband without introducing undesirable artifacts.